1. Field of the Disclosure
The present disclosure relates to a power amplifier and a compound semiconductor device.
2. Description of the Related Art
A mobile communication system is now going to transit from the fourth generation (4G) to the fifth generation (5G). The mobile communication system of the fifth generation additionally uses a higher frequency band than in the fourth generation. As frequency becomes higher, a power loss in a radio frequency circuit increases. Therefore, a higher gain and a higher output are strongly demanded for a radio-frequency power amplifier that is one of the main components of the fifth-generation communication devices (e.g., a cellular phone).
Japanese Unexamined Patent Application Publication No. 2010-278521 discloses a power amplifier in which an initial-stage amplifier element and a subsequent-stage amplifier element are formed on the same GaAs substrate. The initial-stage amplifier element is an enhancement-type field effect transistor, and the subsequent-stage amplifier element is a heterojunction bipolar transistor.
In the power amplifier disclosed in Japanese Unexamined Patent Application Publication No. 2010-278521, the enhancement-type field effect transistor is used as the initial-stage amplifier element with intent to operate the power amplifier only with a positive power supply and to eliminate a negative power supply.
Generally, in an enhancement-type field effect transistor using a compound semiconductor, even when a gate voltage is set to 0 V, a drain current Idss flows in a non-negligible amount (e.g., about 1×10−6 A), and it cannot be completely cut off. The drain current Idss increases at higher temperature. This results in a problem that the power consumption of a battery in a communication device increases. With an increase in power consumption of the battery, a talk time is shortened.